High power fiber laser system with distributive mode absorber

Abstract

A clad absorber unit is provided on a passive fiber of a high power fiber laser system and operative to trap and remove modes propagating along the waveguide clad of the fiber. The mode absorber is configured with such an optimal length that the clad light may be removed in a localized manner, substantially uniformly removed over the entire length thereof. The absorber removing clad light in a unformed fashion includes a host material impregnated with diffusers.

The invention claimed is: 
     
       1. A clad mode stripper (“CMS”) unit for a high power fiber laser system, comprising:
 a passive fiber configured with a core, at least one waveguide cladding surrounding the core, and a polymeric sheath coated upon the cladding and having a discontinuous surface which defines an opening terminating in a plane of the cladding; and 
 a clad mode stripper (“CMS”) filling the opening so as to cover an exposed portion of the cladding, the CMS being configured to provide a substantially uniform removal of a multimode (“MM”) light from the cladding along an entire length of the CMS at a maximum dissipated light power heating the CMS at a temperature lower than a thermal threshold at which the CMS is damaged. 
 
     
     
       2. The CMS unit of  claim 1 , wherein the CMS includes a host material with a refractive index lower than that one of the cladding and a plurality of diffusers impregnated in the host material and scattering the MM light, which is incident upon the CMS, so that a removal of the MM is substantially uniformly distributed along the entire length of the CMS. 
     
     
       3. The CMS unit of  claim 2 , wherein the diffusers are impregnated at a concentration selected to provide the CMS with the entire length which is optimally dimensioned to provide the uniform MM light removal from the cladding along the length at the maximum dissipated power. 
     
     
       4. The CMS unit of  claim 1 , wherein the host material includes silicone, and the diffusers include aluminum oxide particles. 
     
     
       5. The CMS unit of  claim 2 , wherein the passive fiber has adjacent central uniformly dimensioned and tapered regions, the tapered region at least partially coextending with the CMS and having a cone angle selected so as to provide the CMS with the entire length so optimally dimensioned that the removal of the MM light is substantially uniformly distributed along the length at the maximum dissipated power. 
     
     
       6. The CMS unit of  claim 5 , wherein the passive fiber further has first and second end regions with the first region adjoining the one tapered region, and another tapered region bridging the second end region and central region, the CMS extending between the uniformly dimensioned central and first end regions. 
     
     
       7. The CMS unit of  claim 6 , wherein the first and second regions each have a cross-section smaller than that one of the central region, the first and second end regions having a uniform configuration or different configurations, and the tapered regions being uniformly or non-uniformly configured. 
     
     
       8. The CMS unit of  claim 7 , wherein the one tapered region is longer than the other tapered region. 
     
     
       9. The CMS unit of  claim 5 , wherein the cone angle is selected so that that the CMS is operative to remove low numerical aperture modes of the MM light. 
     
     
       10. The CMS unit of  claim 1  further comprising an absorber located downstream from the CMS and operative to locally remove a remaining portion of the MM light not stripped by the CMS. 
     
     
       11. A method of manufacturing a clad mode absorber unit for a fiber laser system having a plurality of passive fibers each of which is configured with a core, at least one cladding surrounding the core and capable of guiding an undesired multimode (“MM”) light, and a polymeric sheath upon the cladding; the method comprising:
 removing a portion of the sheath at a desired location, thereby forming an opening in the sheath so as to expose a length of the cladding; and 
 configuring a clad mode stripper (“CMS”) with an optimal length dimensioned so that, upon being applied to the cladding within the opening, the CMS is operative to substantially uniformly remove the MM along the optimal length thereof at a maximum dissipated light power lower than a thermal threshold at which the CMS is damaged. 
 
     
     
       12. The method of  claim 11 , wherein configuring of the CMS includes:
 providing a host material with a refractive index lower than that one of the cladding; 
 impregnating the host material with light scattering diffusers, and 
 selecting a concentration of the diffusers so as to provide the CMS with the optimal length over which the diffusers substantially uniformly scatter the MM light at the maximum dissipating light power. 
 
     
     
       13. The method of  claim 12 , wherein the host material includes silicone and the scattering diffusers include AL2O3 particles. 
     
     
       14. The method of  claim 11 , wherein providing the passive fiber includes drawing the core and cladding with a uniformly configured central region and at least one tapered region running from one end of the central region at a desired cone angle is selected to provide the CMS with the optimal length. 
     
     
       15. The method of  claim 14 , wherein the cone angle is selected to increase an angle of low numerical aperture (“LNA”) modes of the MM light to a desired angle at which the LNA modes are removed from the cladding. 
     
     
       16. The method of  claim 14 , wherein providing the passive fiber includes drawing the core and cladding with first and second spaced end regions with the first end region adjoining the one tapered region, and another tapered region bridging the second end and central end regions, the end regions each having a substantially uniform cross-section smaller than that one of the central region. 
     
     
       17. The method of  claim 16 , wherein the protective sheath is removed between the central and first end regions so that the CMS extends along the one tapered region either over a full length thereof or a portion thereof. 
     
     
       18. A high power fiber laser system, comprising:
 at least one gain block operative to emit a high power radiation; 
 a plurality of passive fibers optically coupled to the gain block, the passive fiber each having a core, at least one cladding surrounding the core and configured to guide multimode (“MM”) light, and a sheath upon the cladding, at least one of the passive fibers having a part of the sheath removed so as to expose the cladding; 
 a clad mode stripper (“CMS”) applied to the exposed cladding and configured to provide a substantially uniform removal of the MM light from the cladding along an entire length of the CMS at a maximum dissipated light power heating the CMS at a temperature lower than a thermal threshold at which the CMS is damaged. 
 
     
     
       19. The high power fiber laser system of  claim 18  further comprising an additional gain block and at least one pump configured to pump the one gain block in a direction counter to a direction of propagation of signal light, the gain blocks defining a MOPA configuration, wherein the one gain block is configured as a SM power amplifier and the additional gain block includes a SM master oscillator, the passive fiber configured with CMS being a delivery fiber or a fiber between one and additional gain block. 
     
     
       20. The high power system of  claim 18  wherein the gain block is operative to emit pulsed high power radiation or continuous high power radiation, the radiation being emitted in a single mode or multiple modes. 
     
     
       21. The high power system of  claim 18 , wherein the CMS includes a host material with a refractive index lower than that one of the cladding and a plurality of diffusers impregnated in the host material to scatter the MM light incident upon the CMS so that a removal of the MM is substantially uniformly distributed along the entire length of the CMS. 
     
     
       22. The high power system of  claim 21 , wherein the diffusers are impregnated at a concentration selected to provide the CMS with the entire length which is optimally dimensioned to provide the uniform MM light removal from the cladding along the length at the maximum dissipated power. 
     
     
       23. The high power system of  claim 21 , wherein the host material includes silicone, and the diffusers include aluminum oxide particles. 
     
     
       24. The high power system of  claim 21 , wherein the diffusers each are dimensioned so as to provide a substantially unidirectional scattering of the MM light into the host material. 
     
     
       25. The high power system of  claim 19 , wherein the passive fiber has adjacent central uniformly dimensioned and tapered regions, the tapered region at least partially coextending with the CMS and having a cone angle selected so as to provide the CMS with the entire length so optimally dimensioned that the removal of the MM light is substantially uniformly distributed along the length at the maximum dissipated power. 
     
     
       26. The high power system of  claim 25 , wherein the passive fiber further first and second end regions with the first region adjoining the one tapered region, and another tapered region bridging the second end region and central region, the CMS extending between the uniformly dimensioned and first end regions. 
     
     
       27. The high power system of  claim 26 , wherein the first and second end regions each have a cross-section smaller than that one of the central region, the first and second end regions having a uniform configuration or different configurations, and the tapered regions being uniformly or non-uniformly configured. 
     
     
       28. The high power system of  claim 26 , wherein the one tapered region is longer than the other tapered region. 
     
     
       29. The high power system of  claim 25 , wherein the cone angle is selected so that that the CMS is operative to remove low numerical aperture modes of the MM light.